A measuring cell of the kind described above is disclosed in published British patent application No. 2,094,005.
The known measuring cell operates according to the amperometric principle pursuant to which the gas molecules diffusing through the membrane initiate an electrochemical reaction in the electrolyte. This reaction is processed to a measuring signal via the measuring tap on the electrodes. The conversion rate of the gas sample is dependent on temperature so that the sensitivity of the sensors can change with the temperature. This temperature dependency is often also dependent on the age of the sensors and also for the same types of sensors; that is, the temperature dependency can be different from one measuring cell to the next where the sensor configuration (electrolyte and electrode material) is suitable for detecting a specific type of gas.
For obtaining a constant sensitivity, it is known to heat the sensors and to hold the same at a constant temperature. However, this method has the disadvantage that the energy needed therefor, for example, in portable gas measuring apparatus must be supplied by a battery and therefore is limited.
A further known method includes mounting a temperature sensor at a suitable location and processing the temperature signal thereof either in an analog manner in a preamplifier or digitally in a software supported microprocessor while correspondingly compensating the measuring signal (temperature compensation method). In this method, it is disadvantageous that the temperature sensor cannot be mounted in the direct vicinity of the subassembly causing the temperature movement. This subassembly comprises the porous membrane, the catalytic measuring electrode and the electrolyte. A thermal coupling of the temperature sensor to this subassembly is only partially achieved.
Unavoidable temperature gradients are produced by large temperature jumps since the portable apparatus are utilized at different changing locations. These temperature gradients lead to impermissible deviations since a temperature compensation within the measuring cell takes a substantially longer time duration than is required for the measuring operation itself. The measuring operation is carried out with a measuring cell in these cases which is not in thermal equilibrium to the temperature of the gas to be measured. For known measuring cells, a diffusion barrier is interposed over the membrane in the direction facing toward the ambient. This diffusion membrane on the one hand moderates the flow of the sample of the gas to be measured while, on the other hand, is intended to provide a limitation of the diffusion flow because of its configuration in combination with the characteristics of the measuring cell.